Several examples of air drying arrangements are shown and disclosed in Japanese Utility Model Registration No. 55-23621, in Japanese Utility Model Registration No. 59-86231, and in Japanese Pat. No. 53-34664, as well as in our U.S. patent applicaton Ser. No. 932,121, filed Nov. 19, 1986, now U.S. Pat. No. 4,721,515, issued on Jan. 26, 1988, entitled "Control System of a Dual-Cylinder Type Dehumidifier". One control method of switching a pair of drying cylinders into either a drying mode or a regeneration mode is illustrated in FIG. 4 of the present application. In this first control method, a pair of electromagnetic valves, represented by characters MV1 and MV2, are switched "ON" and "OFF" at each preset time period T determined by a suitable timer (not indicated). In other words, the drying and regeneration cycles of one pair of drying cylinders, represented by the characters DR1 and DR2, are equally and alternatively repeated during the operation. The second control method of a prior art arrangement is illustrated in FIG. 5 of the present application. The time chart of FIG. 5 is taken from the Table in FIG. 3 of Japanese Pat. No. 53-34664. As shown, the operational relationship of the ON-OFF periods of the governor GO is used to sense or detect the pressure level P in the air reservoir MR. That is, the cycling, namely, the startup and shutdown of the air compressor CO is controlled by the pressure. It will be noted that both of the electromagnetic valves MV1 and MV2 are in an "ON" state when the air compressor CO stops or shuts down, so that both of the drying cylinders DR1 and DR2 switch to a regenerating phase. When the air compressor CO again switches "ON", the electromagnetic valves MV1 and MV2 are switched to opposite states so that the driers DR1 and DR2 are switched.
In the first control method, illustrated in FIG. 4, the drying and regenerating cycles of the driers DR1 and DR2 are alternately repeated. Each of the recurring periods of time T is predetermined by a timer, which is independent of the operation and shutdown of the air compressor CO. Thus, the driers may be switched while the air compressor is stopped, so that the frequency increases. Therefore, the switching mechanism, including the electromagnetic valves MV1, MV2 and their control portions, tend to wear out and the chance of failures increases proportionally which represents a disadvantage and causes problems.
In the second control method, illustrated in FIG. 5, the pressure in the air reservoir MR is raised from atmospheric air to the upper pressure regulating limit value P2 by the governor GO during time t10 in FIG. 5, when the air in the air reservoir MR is consumed while it is compressed from the lower pressure regulating limit value P1 to the upper pressure regulating limit value P2 by the governor GO. It will be seen that the pressure increase or buildup is slow during time t11. At the same time, the drying cylinder DR1 continues the same drying operation during the entire period. This causes the drying cylinder to go beyond the drying limit and the drying performance to greatly decrease, which poses another problem.